OP, this is great advice. If you’re going to have a difficult conversation anyway (which I agree is needed), please tell her clearly that you think he is abusive and controlling and frequently upsets her, even to the point of suicidal thoughts and plans, tell her she deserves better than this and that not all men are like this, and signpost her to Women’s Aid. She might also benefit from individual formal counselling.
I agree it’s a worry that she wants to marry him, as she will be more tied to him financially and in living situation, but to be honest she is already fairly tied to him marriage or not due to having 2 kids with him. So she is already deeply in a controlling abusive relationship. And it will be very hard to face up to the fact that she will never have the happy family she wants. But you are obviously very close so your opinion does matter to her and it’s definitely still worth specifically pointing out control and abuse as a real problem (can’t believe a pp said this woman trapped in domestic abuse with the father of her children doesn’t have “real” problems, this is as real as it gets),
I have been in a controlling abusive marriage and it really helped me when people pointed out my ex’s awful behaviour to me, specifically stating that it was abusive or controlling (this was incredibly rare despite how awful he was, even after I told people he had punched me in the face, in fact I actually found it quite confusing that friends and family were not horrified by that and telling me to LTB, which I’d been half thinking I needed to do at that point, and it made me think perhaps I was overreacting).
It took time for me to wake up to what was going on and act on it, it’s extremely confusing being in an abusive relationship, and there are odd dynamics in play and considerations you have to make that you can’t really understand until you have been in one I think. It sounds so simple in theory but it’s so hard to extricate yourself in reality. But it’s crucial to have “outsiders” (to the relationship) who are on your side and willing to call out behaviour and label it as controlling or abusive, as that is hard to ignore over time, especially when repeated comments from different sources are made.
Even though it might seem like your words are falling on deaf ears at the time, somewhere in their psyche it will be sinking in deeply. I still remember a few “flashbulb” moments that really changed how I looked at things, and ultimately led to me separating and divorcing my abusive ex husband, including a sharp-eyed work colleague pulling me aside when I was pregnant and pointing out fingerprints bruises on my arm and asking if my husband had grabbed me roughly there. Of course I denied it at the time even though that’s exactly what had happened, I hadn’t realised I had bruises. But she clearly wasn’t convinced and said ok but if that was happening just know that’s not ok, it’s not my fault, it’s not a normal part of a marriage, and that domestic abuse often starts or ramps up in pregnancy and to please come to her if I ever need to speak to someone about it or need financial or practical support in leaving. I’ll be forever grateful to her as she really made me stop and think actually this isn’t ok, other people can see it too, and I’ve literally had to lie to someone’s face about the reality of my marriage, and why did I feel the need to to do that etc. She basically triggered me to admit to myself that I was in an abusive relationship.
Another moment was when a fairly new acquaintance who had been in an abusive relationship herself said she wasn’t at all surprised that my husband picked a huge argument with me about absolutely nothing just before me and her were due to go out for the evening, and suggested he’d done it on purpose to try and stop me going or at least make me so upset and exhausted that I’d struggle to enjoy it properly anyway and build up friendships. I hadn’t ever looked at our arguments that way and it was illuminating as it actually made so much more logical sense of the crazy argument we’d just had over nothing. It’s possible that there is some of this dynamic happening in your situation too, he will want to cut her off from such a close relationship with you so maybe he is picking fights with her on purpose when she is due to see you.
OP I love that you are there for your friend, you describe it as a very close relationship, almost like sisters. Yes you still need to have boundaries to protect your own wellbeing, but it’s very understandable that you find it hard to draw those in such a close relationship with someone with mental health issues and who is currently embroiled in an abusive relationship. And it doesn’t make you weak or stupid etc. Having said that I would really encourage you not to cut off your friend completely, as abusers often successfully end up isolating their victim meaning there is less chance for “outsiders” to notice what is going on and question it etc. It’s so important for her to have someone in touch with her regularly who is on her side. Fell free to tell her you have your own stuff to handle too and can’t spend all your time and energy on her fallouts with her fiancé as you’re finding it really draining of your own spirits, and also want to spend more of your time together making positive memories and having some fun. But do also tell her that you love her and care about her and are ultimately there for her even at 3am in a real crisis, and that you want her to know you see him as controlling and abusive of her.
I’m not surprised she is having these long circular exhausting conversations with you about each argument, one of the tactics of abusers is literally called “crazymaking”, they deliberately cause someone to be in a confused state of brain fog where they are spending so much time and energy trying to just understand what is going wrong in their relationship without ever allowing them to wake up to the simple reality that it’s an abusive relationship and therefore there is nothing they could ever do or say to lead to a different better relationship in future.
What you describe of her relationship is the “cycle of abuse” where the abuse ramps up until there is a huge conflict and eg they break up, followed by a honeymoon period where they get back together and then it all builds again, the cycle tends to get shorter and shorter as time goes on leading to more frequent conflicts and less honeymoon period. It’s well worth educating yourself about the tactics of abusers so that you can spot them and call them out as you see them. There is a book called “Living with the Dominator” that is invaluable for this.